In recent years, the demand for a clean energy increases due to an environmental pollution and a fossil energy exhaustion. In particular, development for a solar cell, which does not cause environment pollution and is a limitless energy supply source, is widely and intensively performed.
Depending on the material of a light absorption layer, the solar cell might be classified into an inorganic solar cell formed of an inorganic material such as silicon or a multinary compound semiconductor, a dye-sensitized solar cell principally formed of an organic material, and an organic solar cell. The solar cell made of crystalline silicon occupies more than 90% in a market share of solar cells. However the manufacture unit cost increases in recent years due to a demand and supply problem in a silicon material, so a power generation costs more as compared to other recycling energy. Therefore the demand for the development of a thin film solar cell, which might be manufactured in a thin film type on a cheap substrate like a glass, a metal, and a plastic instead of an expensive silicon substrate, is urgently needed.
The above thin film solar cell generally uses a substrate made of a cheap flat glass, namely, a soda lime glass. On the soda lime glass substrate, a light absorption layer including amorphous silicon, a multinary compound semiconductor like a CdTe or CIS compound, namely, I-III-VI2 group chalcopyrite is stacked.
In the thin film solar cells, the CIS thin film solar cell, which uses I-III-VI2 group chalcopyrite compound semiconductor as a light absorption layer, has a higher light absorption coefficient and higher energy conversion efficiency, so it is being intensively studied in views of a low cost and a high efficiency.
The CIS compound thin film solar cell is made in a structure that a glass substrate, a metallic electrode layer, a CIS light absorption layer, a high resistance buffer layer, a thin film layer of a transparent electrode, and a reflection prevention layer are stacked in order. When a light absorption layer is stacked on a metallic electrode layer, it is heated up to above 500° C. Alkali components from a soda lime glass, which is used as a substrate, pass through a metallic electrode layer during a heat treatment process and are diffused into a light absorption layer. The migration of a proper amount of alkali components helps increasing an electric charge concentration of a thin film or decreasing a structural characteristic change based on a composition change thereby enhancing the efficiency of a thin film solar cell. That is to say there needs two parameters to achieve an economically benefit and competitive energy conversion efficiency in manufacturing CIS compounds solar cell. They are a high processing temperature more than 500° C. and proper alkali contents.
The strain point of a normal soda lime silicate glass is around 500° C., so a processing temperature of 500° C. is realistically not possible in a mass production line which should use big size glass substrate because it can not guarantee a size stability and safety.
However, when it is over migrated, a light absorption layer might be peeled from an interface of a metallic electrode layer. Since alkali is non-uniformly distributed in a light absorption layer, the efficiency might be worsened, and the lifespan is shortened. The above problems might more increase at a process temperature of above 500° C., so increasing the process temperature is limited.
According to the international application WO 2006/062206, a cheap soda lime glass substrate is used so as to form a CIS thin film solar cell. An alkali diffusion protecting layer of such as oxide or nitride is stacked between a glass substrate and a metallic electrode layer. However, the above method needs an additional process for forming an alkali diffusion protecting layer, and it is needed to coat a Na-contained layer for an alkali supply in order to obtain a high efficiency of a solar cell. Namely, two more processes are additionally needed, which leads to increasing the price of a solar cell. Moreover, as mentioned high processing temperature which is necessary for high conversion efficiency will be restricted when use a normal soda lime silicate glass.
According to the U.S. Pat. No. 6,680,266, it discloses an alumino borosilicate glass which has a low alkali or does not have alkali which might be used in a substrate of a solar cell. The alumino borosilicate glass contains B2O3, so it can effectively prevent a heat spreading of alkali, which is possible to happen in a normal soda lime silicate glass substrate, since it has a high heat resistance performance and does not have an alkali metallic component. However it can not be prepared through the float process which is suitable for a mass production, so the unit cost for its manufacture increases. Furthermore, an alkali doping or Na-contained coating might be further needed for supplying Na which is needed so as to manufacture a high efficiency solar cell. A various kinds of method for an alkali doping or Na-contained coating were invented and applied, but the efficiency results have not reach to the results from a normal soda lime silicate glass substrate.
Meanwhile, the electrochromic device uses an electrochromic phenomenon that change colors by means of electric field and is applied to a window such as a smart window, glasses, a mirror and a display etc. The electrochromic device is manufactured as a transparent electrode layer is formed by stacking ITO or another material on a glass substrate, and an electrochromic layer (WxOy, MoxOy, etc), which is a reduction coloring substance, an electrolyte layer, a counter electrode layer (VxOy, NixOy or another material) are stacked on the same. A cheap soda lime glass substrate can be used as a glass substrate of the above electrochromic layer. When a surplus alkali is spread to ITO in the soda lime glass substrate, the transparent electrode layer might be degraded, and the performance of the electrochromic device might decrease.
A glass substrate having a thermal stability to tolerate a heating process of 500° C.˜520° C. or even 600° C. is needed so as to manufacture a thin film solar cell or an electrochromic device which has a good quality, a high operation performance and a high reproduction performance. The glass substrate further needs a function for preventing or controlling an over spreading of alkali components. At the same time the glass substrate needs a property of supplying a proper alkali content enough to be resulted in high energy conversion efficiency in CIS compounds thin film solar cell.